Use safejs for JS panic handling, add JS to coverage reports

[JohnStarich]

Switches to hack-pad/safejs to properly handle all JavaScript panic scenarios in the IndexedDB FS.

Also adds JS to coverage reports, now that wasmbrowsertest supports JS coverage profiles: https://github.com/agnivade/wasmbrowsertest/pull/41

NOTE: Drops support for Go 1.16 and 1.17 to use safejs. Requires a minor version bump v0.x to give adopters a heads up.

• Add .env file for automatic GOOS/GOARCH editor settings
• Handle new req.Listen errors
• Add JS test coverage to report